wlr_scene: Add support for custom swapchains

include/wlr/types/wlr_scene.h

@@ -468,6 +468,13 @@ void wlr_scene_output_set_position(struct wlr_scene_output *scene_output,
 
 struct wlr_scene_output_state_options {
 	struct wlr_scene_timer *timer;
+
+	/**
+	 * Allows use of a custom swapchain. This can be useful when trying out an
+	 * output configuration. The swapchain dimensions must match the respective
+	 * wlr_output_state or output size if not specified.
+	 */
+	struct wlr_swapchain *swapchain;
 };
 
 /**

types/scene/wlr_scene.c

@@ -1803,11 +1803,16 @@ bool wlr_scene_output_build_state(struct wlr_scene_output *scene_output,
 	wlr_damage_ring_set_bounds(&scene_output->damage_ring,
 		render_data.trans_width, render_data.trans_height);
 
-	if (!wlr_output_configure_primary_swapchain(output, state, &output->swapchain)) {
-		return false;
+	struct wlr_swapchain *swapchain = options->swapchain;
+	if (!swapchain) {
+		if (!wlr_output_configure_primary_swapchain(output, state, &output->swapchain)) {
+			return false;
+		}
+
+		swapchain = output->swapchain;
 	}
 
-	struct wlr_buffer *buffer = wlr_swapchain_acquire(output->swapchain, NULL);
+	struct wlr_buffer *buffer = wlr_swapchain_acquire(swapchain, NULL);
 	if (buffer == NULL) {
 		return false;
 	}